Add support for new power scheme

Two power-related proposals were approved not long ago:

http://wiki.openstreetmap.org/wiki/Proposed_features/Substation_refinement
http://wiki.openstreetmap.org/wiki/Proposed_features/Power_generation_refinement

they greatly refine the tagging scheme and fix many ambiguities, so I hope will be adopted as soon as possible. Thus, I propose to add support for them into the style:

- add support for power=plant and power=substation (mapped as power=station before, z>=10)
- add support for power=transformer (this is optional, and is better to be an icon actually), rendered as power=sub_station before (z>=13)
- render generator in a same way as transformer: it was incorrectly rendered as a station (z>=10), while it's essentially comparably small.